In recent years, portable electronic appliances such as notebook-sized personal computer, a cellular phone and a personal digital assistance have spread wide. As an electric source for these portable electronic appliances, a non-aqueous electrolyte secondary battery is widely used. Further, in recent years, a non-aqueous electrolyte secondary battery also attracts attention as a large size electric source for electric vehicles. The positive electrode of a non-aqueous electrolyte secondary battery is made by forming on a collector a layer composed of a composition comprising a positive electrode active material and a binder (which composition and active layer are referred to as “electrode composition” and “active material layer”, respectively, in the present invention). As the positive electrode active material, LiCoO2, LiNiO2 and others are used. These positive electrode active materials are unstable at overcharge, and therefore, the use of an iron compound such as LiFePO4 is examined for a battery of a large capacity for electric vehicles.
Iron compounds used for an electrode active material generally have low electrical conductivity, and therefore, an electrochemical reaction often does not occur with the result of an increase in the internal resistance and reduction in capacity. To provide an electrode active material having good electrical conductivity, a proposal has been made wherein a composite electrode active material having incorporated therein a carbon material having good electrical conductivity is used, or the electrode active material is coated with a carbon material having good electrical conductivity. Another proposal has been made wherein an electrode active material having a reduced particle diameter is used for increasing the surface area contributing an electrochemical reaction [see US 2002-195, 591 A1 (=JP 2001-15111 A) and JP 2003-36889 A].
However, the electrode active material having a reduced particle diameter leads to increase in the surface area with the results of an increase in the amount of binder used for binding electrode active material particles to each other and to a collector. The binder is electrically non-conductive and thus the use of a large amount of binder also leads to an increase in the internal resistance and reduction in capacity. To obtain an enhanced binding force with a small amount of binder, a proposal has been made which uses a synthetic rubber latex binder [US 2004-121232 A1 (=JP 2004-55493 A)]. The synthetic rubber latex binder includes, for example, styrene-butadiene rubber latex, nitrile-butadiene rubber latex and methyl methacrylate-butadiene rubber latex.
Even with these synthetic rubber latex binders, the electrical conductivity is not enhanced to the desired extent. Further, these rubber latex binders tend to be degraded at a high voltage exceeding 4.0 volt, and thus, the battery performance is deteriorated at overcharge, and the safety is reduced.